This invention relates to coated cemented carbide articles and the method of making such articles. More particularly, it relates to coated cemented carbide articles in which the cemented carbide substrates are modified to improve the bonding of oxide wear layers thereto.
The use of coated cemented carbides for cutting tools is well-known. The prior art coated cemented carbides generally offer a combination of high wear-resistance, mechanical strength and hardness. However, tool material failure sometimes occurs in rigorous applications, such as earth drilling and high speed metal cutting. For instance, although a coating, such as aluminum oxide (Al.sub.2 O.sub.3), increases the wear resistance of tools, inadequate adherence of the coating to the underlying carbide substrate tends to nullify the effect of that improvement, especially in high-speed cutting applications.
Another factor contributing to the failure of coated cemented carbide tools is that increased wear resistance from coatings of titanium carbide (TiC) and aluminum oxide (Al.sub.2 O.sub.3) results in a decrease in the strength of the substrate. Breakage or chipping of the cutting tool accompanies this reduced substrate strength, and tool performance is thus adversely affected. Furthermore, the need for frequent replacement of worn or chipped tools results in higher overall machining costs and lower machine shop productivity.
In U.S. Pat. No. 4,018,631, improved adherence of oxide coatings on cemented carbide substrates containing tungsten and cobalt is attained by forming on the substrate a coating selected from carbides, nitrides, carbonitrides and mixtures thereof, diffusing tungsten and cobalt from the substrate into the coating, oxidizing the coating and then coating the oxidized coating with an oxide wear layer. Although this technique improves adherence of oxide coatings to cemented carbide substrates, and good wear resistance is realized in articles formed by the process of U.S. Pat. No. 4,018,631, it is desirable to further improve the wear resistance of such articles and to improve the adherence of oxide coatings on cemented carbide substrates.
In U.S. Pat. No. 4,277,283, there is disclosed a method for producing a sintered hard metal. The method generally involves the compacting of mixtures of carbides and nitrides of Group IVa, Va and VIa metallic elements, as well as a tungsten carbide phase, into a predetermined shape, and then sintering the pressed mixture in a vacuum at high temperatures. Although it is disclosed in U.S. Pat. No. 4,277,283 that this technique slightly improves the wear resistance of the resulting article, still further enhancement of wear resistance for these types of metals is desirable.
The most effective prior art processes generally involve three steps. For instance, in U.S. Pat. No. 4,018,631, the steps comprise: treating the insert in a chemical vapor deposition (CVD) atmosphere to form carbides, nitrides and carbonitrides of metals, such as titanium and tantalum; firing the insert in a non-reactive atmosphere to diffuse tungsten and cobalt into the CVD-formed coating; and heating the insert in an oxidizing atmosphere to oxidize the coating. Since these three steps require four separate furnace cycles, processing and handling time requirements can result in high production costs.
It is therefore an object of this invention to provide a process to improve the adherence of oxide wear coatings to an underlying tungsten carbide substrate.
It is a further object of this invention to provide a process for increasing the strength of carbide substrates so as to reduce breaking or chipping of cutting tools having wear resistant coatings thereon.
It is still a further object of this invention to provide an improved process for pretreating and oxidizing a cutting tool substrate which requires a smaller number of furnace cycles than in prior art processes.
It is yet another object of this invention to provide an improved process for pretreating and oxidizing a cutting tool substrate which is not subject to the above-mentioned problems and disadvantages.